1. Field of the Disclosure
This disclosure relates generally to the reading out of image sensors, and in particular but not exclusively, relates to the reading out of image sensors that include shared charge-to-voltage conversion mechanisms.
2. Background
Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal-oxide-semiconductor (“CMOS”) image sensors, has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.
CMOS image sensors can use arrays of pixels. A pixel generally includes a photodiode (i.e., a photo-sensitive region responsible for collecting electromagnetic energy and converting the collected electromagnetic energy into electrons), a transfer transistor, a source follower amplifier transistor, and a row select transistor. Photo-generated charge accumulated in the photosensitive region of the photodiode is ultimately converted into a voltage by a charge-to-voltage mechanism (also referred to as a floating diffusion) included in the pixel.
Some image sensors may include a shared pixel architecture, where photodiodes are grouped together to form pixel units that have a shared charge-to-voltage mechanism, as well as one or more shared transistors, among several photodiodes.
The efficiency with which the photosensitive regions of a pixel unit converts incident electromagnetic energy into accumulated electrons depends on many factors, including the full well capacity (FWC) of the photodiodes. FWC is a measure of the number of electrons a photodiode can store before it reaches saturation. When the saturation of a photodiode is reached, excess electrons may overflow to adjacent pixels. Increased photodiode FWC may result in a higher dynamic range and higher signal-to-noise ratio for a CMOS sensor, which ultimately results in higher-quality digital images.
However, an increase in the photodiode FWC may result in an increase in the occurrence of “black dots” in the resultant image. This is because an increased FWC would require a charge-to-voltage conversion region that is reset to a larger voltage to accommodate a larger voltage swing. This larger voltage potential on the charge-to-voltage mechanism may cause one or more of the pixel transistors (e.g., source-follower transistor) to operate outside of its linear operation region. The non-linear operation of the source follower transistor may cause a signal level, in low light conditions (i.e., high voltage on the floating diffusion), to be compressed which results in the increased occurrence of the black dots.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.